A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field or the main magnetic field.
During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used. The transmitted RF field is referred to as the B1 field.
However, spurious RF noise during the MRI scan can interfere with the measurement of the RF signals emitted by the nuclear spins. Typically, a large RF cage is built around the magnetic resonance imaging system to eliminate these spurious RF signals. A disadvantage to using such an RF cage is that it uses large amounts of metal such as copper and is expensive to build.
U.S. Pat. No. 7,486,982 B2 discloses a radio-opaque holder in combination with radio-opaque magnet components to form an RF shield around a patient undergoing an NMR procedure.
JPS63272336A discloses two sets of external radiowave detection coils that are arranged in the vicinity of the MR signal detection coil but substantionally do not perform the reception of the MR signal from the examinee. The effect of the external radiowave is removed by directly subtracting the receiving signal of the external radiowave detection coils from that of the MR signal detection coil. It is understood that the external radiowave detections coils have to be carefully placed relative to the signal detection coil to ensure the effect of noise cancellation. The international application WO2013/016639 discloses an active noise cancellation system in a portable MR system, which calculates a universal complex scaling factor and determines a transfer function between environmental spurious noise and the measured signal from the receiver coil, which is herein incorporated by reference in its entirety.